1. Field
The present disclosure relates generally to energy storage devices, and more particularly to supercapacitor technology and the like.
2. Background
The success of a future energy-efficient economy largely depends on our ability to develop novel materials with greatly improved characteristics for electrical energy storage and delivery. FIG. 1 is a chart illustrating various power and energy densities in modern electrical energy storage devices. For reasons evident here, supercapacitors have recently attracted much attention and are taking on increased importance in applications where long operational lifetime, high power, and low weight are essential, including hybrid electrical vehicles (HEV), energy-efficient hybrid industrial equipment, elevators, mobile electronic devices, power quality devices, current instability leveling devices in smart electrical grids, and peak power sources for military applications. Supercapacitors may also be useful in delivering high currents for rapid heating or quick high-intensity lighting (e.g., flashes in cameras, on-demand night lighting for roads, etc.), sending strong electromagnetic radio signals, triggering chemical reactions, and even launching rockets. They may also greatly increase the lifetime of batteries or fuel cells if used jointly with such energy storage/conversion devices.
Supercapacitors are rechargeable electrochemical energy storage devices similar to batteries, but with different performance characteristics. Supercapacitors can store more energy in a smaller volume and often at a lower cost. In contrast to batteries, supercapacitors may operate efficiently in a large temperature window (e.g., from about −40 to +300° C., depending on the electrolyte used), have an incredibly long cycle life (typically greater than 100,000 cycles), and can often be charged in less than about a second. Such properties are unattainable in Li-ion batteries, for example. Additionally, in contrast to Li-ion batteries, open damage to the cell of a supercapacitor does not cause a fire. They are also much more environmentally friendly.
There are generally two types of supercapacitors that are commercially available. FIG. 2 illustrates a first type of commercially available supercapacitor, namely a traditional electrical double layer capacitor (EDLC) with carbon electrodes. The charge storage mechanism in a pure EDLC is non-Faradaic. During charging and discharging of an EDLC, no charge transfer takes place across the electrode/electrolyte interface and the energy storage is electrostatic in nature. As shown, an EDLC consists of two electrodes 202, 204 immersed in an electrolyte 206 and separated by an ion-conducting but electron-blocking membrane 208. Upon application of an electrical potential to one of the electrodes, ions of opposite sign accumulate on the electrode surface in a quantity proportional to the applied voltage, forming a so-called electrical double layer. This double layer consists of an electrical space charge from the electrode side and an ion space charge from the electrolyte side. The values for the double layer capacitance normalized per unit area generally vary from about 3 to about 30 μF/cm2.
A second type of commercially available supercapacitor is a so-called pseudocapacitor with expensive hydrous ruthenium oxide (RuO2.nH2O) electrodes. While pseudocapacitors based on conductive metal oxides (such as RuOx) and conductive polymers may offer higher capacitance per unit mass, they also suffer from shorter lifetimes, higher costs, and most importantly, a lower operational voltage range. Since common organic electrolytes are mostly not electrochemically (Faradaically) active with metal oxides or polymers, pseudocapacitors are instead typically built with aqueous electrolytes. This leads to a lower operational voltage, however, due to the decomposition of H2O in aqueous electrolytes at voltages in excess of 1V.
Carbon-based supercapacitors (EDLC) are therefore believed to offer a more practical solution for many energy storage applications. Since the energy storage of a supercapacitor is proportional to the square of the voltage (E=0.5 C·V2), increasing the voltage from 1V (aqueous electrolytes commonly used in pseudocapacitors) to about 2.5V-4V (organic electrolytes or ionic liquids commonly used in carbon-based EDLCs) results in higher energy density in spite of the lower capacitance of carbon. An additional advantage of carbon-based supercapacitors is their much faster charge-discharge kinetics.
Nevertheless, there remains a need for improved carbon-based supercapacitors, components, and other related materials and manufacturing processes thereof.